This invention has application in the field of static separators of the spiral type fitted in vertical tubular conduits, preferably at the bottom of oil wells.
1. Prior Art
The oil produced from wells in productive oilfields is always accompanied by a gas fraction forming a two phase fluid mixture. This gas either comes out of solution when the pressure is reduced as the mixture flows from the reservoir to the well, or is already present in the form of free gas at the reservoir pressure and temperature.
When the pressure in a stratum of oil is not sufficient to raise the fluids produced up to the surface with the desired rate of flow, production wells are equipped with means for effecting some method of artificial lifting.
Depending on the characteristics of the well and of the oil produced, the pumps used for pumping from below ground by one of said methods of artificial lifting may be (i) of the SCP (submerged centrifugal pumping) type, using multiple stage centrifugal pumps, or (ii) of the PCP (progressive cavity pumping) type using volumetric pumps, or (iii) of the MP (mechanical pumping) type using volumetric pumps of the suction type.
Entry of gas into any of these pumps reduces the volumetric efficiency of pumping in pumps of the alternating or progressive cavity type and causes cavitation in centrifugal pumps, to the extent that the fluid suction is reduced to zero if there is, in the pump inlet, in excess of a particular volumetric percentage of free gas which percentage is specific to each pumping system. The existence of problems of the nature of those mentioned above which can occur in pumping systems brings about a drastic decrease in the time between pump failures, significantly increasing the operating costs of these systems.
Multistage separators are used at the bottom of oil wells to avoid or minimize the entry of free gas into a pump, maximizing its volumetric pumping efficiency.
There is abundant literature on designs of separators for use upstream of bottom-hole pumps, with a profusion of patents and articles concerning the subject, indicating that no separator with a satisfactory performance has yet been obtained. Some of the separators use one or several spiral channels to let down the fluid, mounted internally to make use of the centrifugal effect and the increase in the fluid""s residence time within the separator, which implies an increase in separation efficiency. One example of a type of spiral separator can be found in U.S. Pat. No. 5,570,744 by the Atlantic Richfield Company. Nevertheless, in general these separators have a chronic problem with trapping of the gas which separates out within the channels due to flooding of the fluid inlet region. U.S. Pat. No. 5,698,014 by the same Atlantic Richfield Company describes a possible solution to the abovementioned problem with the inclusion, in a separator, of an element which stops the flow of fluid in the gas discharge duct and is connected to a floating actuating member, which closes off the inlet duct whenever the build-up of liquid reaches a very high level within the said separator, using cooperating telescopic members linked to the support of the stop member and the floating actuator member, with a spring placed between them to absorb impacts, but with an increase in failure-prone mechanical devices.
2. Object of the Invention
It is an object of this invention to provide a spiral separator at the bottom of an oil producing well with a channel for the escape of gas, which has the function of providing a free path for the flow of gas which is occasionally trapped in the separating channel due to possible flooding in the vicinity of the separator inlet.
A first aspect of this invention relates to a bottom-hole spiral separator in an oil production well with a channel for the escape of gas which has the function of providing a free path for the flow of gas which is trapped from time to time in the separating channels due to possible flooding of the separator inlet region.
The fluid which is to be separated is collected or transported, by means of a helical capture device along which separation takes place, to an inlet pipe connected to a pump. The introduction of another section which is identical to that of the helicoidal capture device provides a kind of channel for the escape of separated gas, which can sometimes become trapped when the inlet region for the fluid which is to be separated becomes flooded.
The invention also provides a method of separating a multi-phase fluid flow from an oil producing well into a liquid phase and a gaseous phase, comprising placing a spiral separator according to the first aspect, such that the multiphase fluid enters said spiral separator through the inlet region and enters via a separating channel,
causing the multiphase fluid to flow through said separating channel as a result of the suction from a pump connected to the production pipe, and
operating the pump of a rate such that the velocity of the multiphase fluid flowing through helicoidal separating channel provides sufficient centrifugal force for the gaseous phase to be separated from the liquid phase,
wherein once the liquid phase has been separated from the gaseous phase the gaseous phase coalesces and rises, and accumulates in the upper region of the separating channel,
wherein the more fluid phase continues on its path in the direction of the inner lower part of said shell of the spiral separator, accumulates in the fluid phase region, is captured by production pipe and is conveyed to the pump inlet, and
wherein the gaseous phase enters through the gas channel and flows to its outlet at the top of the extension of the gas channel above the inlet region for multiphase fluid.